引用到std :: map有什么问题？ [英] What is wrong with reference into std::map?

问题描述

预期的节目输出：

键是：0字符串是：你好

键是：1字符串是：再见

键是：2字符串是：结束


实际输出：

键是：0字符串是：结束

键是： 1字符串是：

键是：2字符串是：


我做错了什么？


如果我每次都声明我的参考新鲜，比如把{}放在我插入的

个地方并做

std :: string& MyString =（* it）.second;

每次出来都正确。


为什么我不能重复使用这些参考文件？


花了很长时间才发现我的程序中导致这种情况的原因。这个

只是一个显示问题的测试程序。


#include< iostream>

#include< string>

#include< map>


std :: map< unsigned int，std :: string> MyMap;


int main（）

{

unsigned int ID = 0;

std ::地图< unsigned int，std :: string> :: iterator it = MyMap.insert（

MyMap.end（），std :: make_pair< unsigned int，std :: string>（ID，

std :: string（）））;

std :: string& MyString =（* it）.second;

MyString =" Hello";


ID = 1;

it = MyMap.insert（MyMap.end（），std :: make_pair< unsigned int，

std :: string>（ID，std :: string（）））;

std :: string& MyString2 =（* it）.second;

MyString =" Goodbye";


ID = 2;

it = MyMap.insert（MyMap.end（），std :: make_pair< unsigned int，

std :: string>（ID，std :: string（）））;

MyString =（* it）.second;

MyString =" The End";


for（std :: map< unsigned int，std :: string> :: iterator i = MyMap.begin（）;

i！= MyMap.end（）; ++ i）

{

std :: cout<< 关键是： << （* i）.first<< "字符串是： <<

（* i）.second<< std :: endl;

}


std :: string wait;

std :: cin>>等待;

}

Expected output of program:
Key is: 0 String is: Hello
Key is: 1 String is: Goodbye
Key is: 2 String is: The end

Actual output:
Key is: 0 String is: The End
Key is: 1 String is:
Key is: 2 String is:

What am I doing wrong?

If I declare my reference fresh each time, such as putting { } around the
places I insert and doing
std::string& MyString = (*it).second;
each time it will come out right.

Why can''t I reuse the references?

It took a long time to find out what was causing this in my program. This
is just a test program showing the issue.

#include <iostream>
#include <string>
#include <map>

std::map<unsigned int, std::string> MyMap;

int main ()
{
unsigned int ID = 0;
std::map< unsigned int, std::string>::iterator it = MyMap.insert(
MyMap.end(), std::make_pair< unsigned int, std::string >( ID,
std::string() ) );
std::string& MyString = (*it).second;
MyString = "Hello";

ID = 1;
it = MyMap.insert( MyMap.end(), std::make_pair< unsigned int,
std::string >( ID, std::string() ) );
std::string& MyString2 = (*it).second;
MyString = "Goodbye";

ID = 2;
it = MyMap.insert( MyMap.end(), std::make_pair< unsigned int,
std::string >( ID, std::string() ) );
MyString = (*it).second;
MyString = "The End";

for ( std::map< unsigned int, std::string >::iterator i = MyMap.begin();
i != MyMap.end(); ++i )
{
std::cout << "Key is: " << (*i).first << " String is: " <<
(*i).second << std::endl;
}

std::string wait;
std::cin >> wait;
}

推荐答案

"吉姆兰斯顿 < TA ******* @ rocketmail.com>在消息中写道

新闻：TB ************* @ fe04.lga ...

"Jim Langston" <ta*******@rocketmail.com> wrote in message
news:TB*************@fe04.lga...

预期的节目输出：
关键是：0字符串是：你好
关键是：1字符串是：再见
关键是：2字符串是：结束

实际输出：
键是：0字符串是：结束
键是：1字符串是：
键是：2字符串是：

我做错了什么？
std :: string&的位置。 MyString =（* it）.second;
每次它都会正确显示。

为什么我不能重复使用这些参考文件？

它需要一个很长一段时间来找出我的程序中导致这种情况的原因。这个
只是一个显示问题的测试程序。

#include< iostream>
#include< string>
#include< map>

std :: map< unsigned int，std :: string> MyMap;

int main（）
{
unsigned int ID = 0;
std :: map< unsigned int，std :: string> :: iterator it = MyMap.insert（
MyMap.end（），std :: make_pair< unsigned int，std :: string>（ID，
std： ：string（）））;
std :: string& MyString =（* it）.second;
MyString =" Hello";

ID = 1;
它= MyMap.insert（MyMap.end（），std： ：make_pair< unsigned int，
std :: string>（ID，std :: string（）））;
std :: string& MyString2 =（* it）.second;


这是一次破碎的诊断尝试。即使这一行更改为：

std :: string& MyString2 =（* it）.second


输出相同。

MyString =" Goodbye";

ID = 2;
它= MyMap.insert（MyMap.end（），std :: make_pair< unsigned int，
std :: string>（ID，std :: string（）））;
MyString =（* it）.second;
MyString =" The End" ;;

for（std :: map< unsigned int，std :: string> :: iterator i =
MyMap.begin（）; i！= MyMap.end（）; ++ i）
{
std :: cout<< 关键是： << （* i）.first<< "字符串是： <<
（* i）.second<< std :: endl;
}

std :: string wait;
std :: cin>>等;;

Expected output of program:
Key is: 0 String is: Hello
Key is: 1 String is: Goodbye
Key is: 2 String is: The end

Actual output:
Key is: 0 String is: The End
Key is: 1 String is:
Key is: 2 String is:

What am I doing wrong?

If I declare my reference fresh each time, such as putting { } around the
places I insert and doing
std::string& MyString = (*it).second;
each time it will come out right.

Why can''t I reuse the references?

It took a long time to find out what was causing this in my program. This
is just a test program showing the issue.

#include <iostream>
#include <string>
#include <map>

std::map<unsigned int, std::string> MyMap;

int main ()
{
unsigned int ID = 0;
std::map< unsigned int, std::string>::iterator it = MyMap.insert(
MyMap.end(), std::make_pair< unsigned int, std::string >( ID,
std::string() ) );
std::string& MyString = (*it).second;
MyString = "Hello";

ID = 1;
it = MyMap.insert( MyMap.end(), std::make_pair< unsigned int,
std::string >( ID, std::string() ) );
std::string& MyString2 = (*it).second;
This was a broken attempt to diagnose. Even with this line changed to:
std::string& MyString2 = (*it).second

the output is the same.
MyString = "Goodbye";

ID = 2;
it = MyMap.insert( MyMap.end(), std::make_pair< unsigned int,
std::string >( ID, std::string() ) );
MyString = (*it).second;
MyString = "The End";

for ( std::map< unsigned int, std::string >::iterator i =
MyMap.begin(); i != MyMap.end(); ++i )
{
std::cout << "Key is: " << (*i).first << " String is: " <<
(*i).second << std::endl;
}

std::string wait;
std::cin >> wait;

Jim Langston写道：

Jim Langston wrote:

std :: string& MyString2 =（* it）.second;
MyString =" Goodbye" ;;


你要踢自己，但是你想要在上面一行的
中使用MyString2。你正在这里插入第一个插入的

字符串。
ID = 2;
它= MyMap.insert（MyMap.end（），std :: make_pair< unsigned int，
std :: string>（ID，std :: string（）））;
MyString =（* it）.second;


这不会初始化引用，它会将第二个

的值分配到MyString初始化的位置（第一次插入）。

MyString =" The End" ;;


这又做了另一个这样的任务

std::string& MyString2 = (*it).second;
MyString = "Goodbye";
You''re going to kick yourself, but you want MyString2 in
the above line. You''re righting into the first inserted
string here.
ID = 2;
it = MyMap.insert( MyMap.end(), std::make_pair< unsigned int,
std::string >( ID, std::string() ) );
MyString = (*it).second;
This doesn''t initialize a reference, it assigns the value of second
into where MyString was initialized (your first insertion).
MyString = "The End";
And this does another such assignment

Ron Natalie < ro*@spamcop.net>在消息中写道

news：44 ************** @ spamcop.net ...

"Ron Natalie" <ro*@spamcop.net> wrote in message
news:44**************@spamcop.net...

Jim Langston写道：

Jim Langston wrote:

std :: string& MyString2 =（* it）.second;
MyString =" Goodbye";
你要踢自己，但是你想要MyString2在上面这一行。你正在这里插入第一个插入的
字符串。

std::string& MyString2 = (*it).second;
MyString = "Goodbye";
You''re going to kick yourself, but you want MyString2 in
the above line. You''re righting into the first inserted
string here.

ID = 2;
它= MyMap.insert（MyMap.end（），std： ：make_pair< unsigned int，
std :: string>（ID，std :: string（）））;
MyString =（* it）.second;

ID = 2;
it = MyMap.insert( MyMap.end(), std::make_pair< unsigned int,
std::string >( ID, std::string() ) );
MyString = (*it).second;

<这不会初始化引用，它会将第二个值分配到MyString初始化的位置（第一次插入）。

This doesn''t initialize a reference, it assigns the value of second
into where MyString was initialized (your first insertion).

哦好吧，拍摄。你是对的。那完全超越了我。如果在同一行声明了引用

，那么它会初始化它。


嗯...那么如何分配已经初始化的引用？

IE

int int1;

int int2;

int& MyRef = int1;

我该怎么做才能让MyRef现在指向int2？或者我可以吗？

Oh, well, shoot. You''re right. That totally excaped me. If the reference
is declared on the same line, then it initializes it.

Hmm.. so how do you assign a reference that has already been initialized?
I.E.
int int1;
int int2;
int& MyRef = int1;
Who do I do to get MyRef to point to int2 now? Or can I?

MyString =" The End";

MyString = "The End";

这会做另一个这样的任务

And this does another such assignment

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